1. Field of the Invention
This invention relates to an electrochemical cell such as a solid-state voltage storage cell using a solid electrolyte, and more particularly, it relates to a solid-state voltage storage cell having excellent high-rate voltage storage characteristics.
2. Description of the Prior Art
In recent years, electrochemical cells using a liquid electrolyte, such as batteries and capacitors, have been widely used. However, such conventional electrochemical cells have a disadvantage in that leakage of the liquid electrolyte or gas generation may occur, and there is a possibility of causing the expansion or rupture of the cells. This introduces a serious problem in that the greatest reliability can never be ensured on the use of such electrochemical cells in electric or electronic apparatus.
To eliminate this problem, solid-state electrochemical cells have been developed which employ a solid electrolyte. In such a solid-state electrochemical cell, there arises no problem such as mentioned above, nor other problems associated with a liquid electrolyte, such as freezing or evaporation of the electrolyte. Therefore, solid-state electrochemical cells using a solid electrolyte can be used over a wide range of temperatures, and can also assure high reliability in the use for electric or electronic apparatus.
For this reason, many attempts have been made to develop solid-state electrochemical cells using a solid electrolyte in place of a liquid electrolyte. In particular, intensive work has been done to develop a solid-state battery using a solid electrolyte. Examples of the solid-state batteries which have been hitherto developed include solid-state secondary batteries using any one of the copper-ion conductive solid electrolyte, silver-ion conductive solid electrolyte, and lithiumion conductive solid electrolyte.
There has also been proposed a solid-state voltage storage cell using a solid electrolyte with extremely small self-discharge. For example, a solid-state voltage storage cell has been developed which includes Ag.sub.2 Se-Ag.sub.3 PO.sub.4 or silver-vanadium compound oxide as an electrode active material and also includes, as a silver-ion conductive solid electrolyte, 4AgI-Ag.sub.2 WO.sub.4 which is stable against water, oxygen and heat, and also has extremely small electron conductivity even at high temperatures. The solid-state voltage storage cell including Ag.sub.2 Se-Ag.sub.3 PO.sub.4 as an electrode active material has been put into practical use.
FIG. 1 shows a solid-state voltage storage cell such as mentioned above. The voltage storage cell comprises a silver-ion conductive solid electrolyte layer 1, and a pair of electrodes 2 disposed so as to interpose the solid electrolyte layer 1. With the use of a carbon paste layer 3, a terminal lead 4 is attached to the major surface of each electrode 2, and the whole surface is coated with a resin 5, except that each terminal lead 4 is left out of the resin coating.
A conventional solid-state voltage storage cell where Ag.sub.2 Se-Ag.sub.3 PO.sub.4 is used in the electrode 2 of the above-described configuration has a disadvantage of storing a voltage only in the narrow range of 0 to 100 mV.
On the other hand, a conventional voltage storage cell where a silver-vanadium compound oxide is used in the electrode 2 can store a voltage in the wider range of 0 to 200 mV, as compared with the conventional voltage storage cell mentioned above. Moreover, as described above, the solid electrolyte used together with this electrode active material is stable against water, oxygen and heat, and has almost no electron conductivity even at high temperatures, so that the voltage storage cell including the silver-vanadium compound oxide as the electrode active material can be used over a wide range of temperatures, e.g., even at temperatures exceeding 100.degree. C.
With the configuration in which the solid electrolyte layer is interposed between the electrodes formed from a mixture of the electron active material and the solid electrolyte, however, the solid-state voltage storage cell has high internal resistance and therefore has low voltage-storing rate (in other words, low charging and discharging rates) because of poor electron conductivity of the electrode active material. In particular, when charged and discharged for a short period of time, the voltage stored in the solid-state voltage storage cell has a tendency to deviate from a given level, thereby making it difficult to obtain a satisfactory response to input signals.